This invention relates to instruments such as analytical instruments for example gas chromatographs that are used in hazardous atmospheres and more particularly to a flow sensor for use in such an instrument to detect purge gas.
Gas chromatographs and other analytical instruments used in hazardous atmospheres are a potential source of explosions. Electrical arcs, sparks, and hot surfaces can cause explosions in the presence of a flammable substance and an oxidizer.
In 1977 CENELEC established EN 50014 as a general set of standards to protect against explosions in potentially explosive atmospheres. Further information about EN 50 014 may be found in the publication of the Technical Committee CENELEC TC 31 entitled xe2x80x9cElectrical apparatus for potentially explosive atmospheres General requirementsxe2x80x9d dated June 1997 and published by the CENELEC Central Secretariat, Rue de Stassart 35, B-1050 Brussels, Belgium. In addition, CENELEC also established EN 50016 to define the use of purged and pressurized equipment as a means of protection.
In 1994 the European Parliament and the Council of the European Union Community enacted Directive 94/9/EC, otherwise known as ATEX, to regulate equipment used in potentially explosive atmospheres. The ATEX Directive requires compliance with the latest CENELEC standards. In 1995, a European harmonized standard was developed and EN 50016 was revised to the current second edition. The second edition of EN 50016 can be found in the publication of SC31-7 of the Technical Committee CENELEC TC 31 entitled xe2x80x9cElectrical apparatus for potentially explosive atmospheresxe2x80x94Pressurized apparatus xe2x80x98pxe2x80x99xe2x80x9d dated October 1995 and published by the CENELEC Central Secretariat.
The first edition of EN 50016 did not allow purging of an enclosure as a means of protection if any source of release of flammable gases was present. The second edition of EN 50016 recognized limited sources of release as potentially safe if certain measures are taken. A dilution area must be established around the potential source of release and no source of ignition can be present. Additionally, the purge flow rate and flow path must be adequate to keep the concentration of flammable gases below 25% of the lower explosive limit.
As is shown in FIG. 1, only using a differential pressure switch 14 that senses overpressure does not ensure that the purge gas is passing through the enclosure 11 and providing an effective purge. Purge gas may leak out near the enclosure inlet 10 and the concentration of flammable gases could increase to a dangerous level. In order to prevent this condition, the second edition of EN 50016 requires in section 5.7 that gas flow be monitored at the enclosure outlet 12. The purge gas flow sensor of the present invention provides an effective, inexpensive and reliable way to meet this requirement.
The present invention is an instrument that has an enclosure that has an opening through which a fluid can flow. The instrument also has first and second differential pressure switches. The instrument further has a sealed chamber in the opening. The chamber has only one inlet, an outlet and only one path between the only one inlet and the outlet through which the fluid can flow. The sealed chamber also has first and second restrictors through which the fluid can flow; and means for transferring the pressure in the sealed chamber to the first and second differential pressure switches, the pressure in the enclosure to the first switch and the pressure at the sealed chamber outlet to the second switch.
The present invention is also a flow sensor for use in an instrument. The flow sensor has first and second differential pressure switches and a sealed chamber. The sealed chamber has only one inlet and an outlet through which a fluid can flow and only one path between the only one inlet and the outlet; a flow restrictor in the only one inlet and a flow restrictor in the outlet; and means for transferring the pressure in the sealed chamber to the first and second differential pressure switches.
The present invention is also a flow sensor for use in an instrument. The flow sensor has a sealed chamber. The sealed chamber has only one inlet, an outlet and only one path between the inlet and the outlet through which the fluid can flow; a flow restrictor in the only one inlet and a flow restrictor in the outlet; and means for transferring the pressure in the sealed chamber to first and second differential pressure switches.
The present invention is also the combination of an instrument that has an enclosure having an opening through which a fluid can flow and a flow sensor. The flow sensor has first and second differential pressure switches; and a sealed chamber in the opening, with the chamber having only one inlet, an outlet and only one path between the only one inlet and the outlet through which the fluid can flow. The sealed chamber has first and second restrictors through which the fluid can flow; and means for transferring the pressure in the sealed chamber to the first and second differential pressure switches, the pressure in the enclosure to the first switch and the pressure at the sealed chamber outlet to the second switch.
The present invention is further an instrument that has an enclosure having an opening through which a fluid can flow. The instrument also has a first pressure transducer in the enclosure and a second pressure transducer outside of the enclosure. The instrument further has a sealed chamber in the opening and the chamber has only one inlet, an outlet and only one path between the only one inlet and the outlet through which the fluid can flow. The sealed chamber has first and second flow restrictors through which the fluid can flow. The chamber also has means for transferring the pressure in the sealed chamber to the first and second pressure transducers. The chamber further has means connected to the first and second pressure transducers for calculating for any given rate of flow of the fluid through the sealed chamber the flow through the outlet.
The present invention is a method for detecting the flow of a fluid through an enclosure having an outlet device through which the fluid can flow. The outlet device has a sealed chamber with only one inlet, an outlet, only one path between the only one inlet and the outlet through which the fluid can flow and a flow restrictor in the only one inlet and a flow restrictor in the outlet through which the fluid can flow. The method transfers the pressure in the sealed chamber to first and second differential pressure switches; transfers the pressure in the enclosure to the first switch; and transfers the pressure outside of the enclosure to the second switch.